few fi table doubts

Hi,

can anyone tell what for table bseg and bsak is used ,

i have a select statement like this

1)I KNOW THIS STATEMENT IS TO RETERIVE THE INVOICES

PAID BY CHECK,AND WHAT DOES BSCHL = '31'

iAM NOT SURE WHAT POSTING KEY (bschl= 31) MEANS

SELECT * FROM BSAK WHERE BUKRS = PAYR-ZBUKRS AND

AUGBL = PAYR-VBLNR AND

BSCHL = '31'.

2)CAN ANYONE TELL ME WHAT MIGHT BE THE PURPOSE FOR RETREIVING

THIS DATA for below statement AND WHAT BSCHL = 40 MEANS

SELECT * FROM BSEG WHERE BUKRS = BSAK-BUKRS AND

BELNR = BSAK-BELNR AND

GJAHR = BSAK-GJAHR AND

BSCHL = '40'.

CHECK BSEG-HKONT(4) = '0006'.